In the construction industry, it is always important, and it is often a requirement, to protect construction workers against falling from the structures they are constructing. Such “fall protection” is typically provided by equipping workers with harnesses attached by cables called “lanyards” to anchor points on the structure.
The need for fall protection in the case of high-rise structures is obvious, and a characteristic of such structures is the use of I-beams as structural members. Accordingly, a class of anchoring devices known as “beam anchors” has been provided specifically for utilizing I-beams as anchor points.
FIG. 1 shows a typical beam anchor 1. The beam anchor has a cross-bar 2 and a pair of I-beam capturing members 3, 4 at opposite ends of the cross-bar. The capturing members 3, 4 attach the cross-bar to an I-beam as discussed below in connection with FIG. 2.
A coupler 5 has a circular aperture “A” through which the hook of a lanyard can be attached. Typically, one of the I-beam capturing members 3 is fixedly attached to the cross-bar, while the position of the other capturing member 4 is adjustable along the length of the cross-bar, so that the spacing between the capturing members is adjustable to fit the varying widths of different sized I-beams. A number of different mechanisms may be used to adjust the position of the I-beam capturing member 4 and lock it in place.
For example, as shown in FIG. 1, a spring-biased detent mechanism may be provided that allows for locating the capturing member 4 in spaced increments. Detents “D1” in the cross-bar 2 are shown, while the corresponding spring-biased activating mechanism “D2” in the I-beam capturing member 4 is indicated schematically. Some other well known prior art mechanisms employ holes in the cross-bar and pins extending through the capturing member 4 into a selected one of the holes, or nuts threaded onto the cross-bar.
While it is known to be advantageous to dispose the coupler centrally with respect to the I-beam, so that the load is evenly distributed across the I-beam capturing members 3, 4, this objective has not been achieved in practice. Two approaches have been taken.
In the first approach, the coupler 5 has been fixedly attached to the cross-bar in a central location of the cross-bar relative to the capturing members 3 and 4. However, when one of the capturing members 3, 4 is moved relative to the cross-bar while the position of the other remains fixed, the central location changes. Then, the fixed position of the coupler 5 is no longer a central location relative to the I-beam. Accordingly, the position of the coupler 5 is chosen as a compromise.
In the second approach, the coupler 5 is allowed to slide freely along the cross-bar, thus permitting the coupler 5 to assume the ideal, central location, regardless of the spacing between the capturing members. However, the coupler is also able to slide out of this position.
The problem is avoided if both I-beam capturing members are adjustable and moved the same amount (in opposite directions); however, this requires a more complex and costly device that is more difficult to use.
Another problem with the beam anchor can best be appreciated with reference to FIG. 2, showing a beam anchor attached to an I-beam. The I-beam has a center section 6 whose primary function is to support two spaced-apart flanges 7, 8 which carry the bulk of the bending load. The beam anchor rides on one of the flanges, here the flange 7, in and out of (i.e., perpendicular to) the plane of the Figure. More particularly, over-hanging portions 3a, 4a of the capturing members 3, 4 slide on the upper surface 7a of the flange 7. By riding the flange, the beam anchor can follow the worker as the worker moves along the I-beam.
The capturing members 3, 4 are adjusted as described above to fit the width “W” of the flange 7. There remains a gap “g” between each capturing member 3, 4, and the flange 7 to allow the beam anchor to move without substantial frictional interference with the edges 9 (specifically shown as 9a, 9b) of the flange. However, as a consequence of this gap, the beam anchor 1 is susceptible to twisting about the vertical axis “L1,” i.e. in the plane perpendicular to the plane of the Figure, as it is moved along the length of the I-beam. Referring in addition to FIG. 3, when the beam anchor twists (or rotates) about the axis L by an increasing angle θ, the distance “d” between the capturing members as shown in FIG. 2 becomes less able to span the width “W” of the beam, until d multiplied by cosine(θ) is reduced to the point of equaling W, whereupon an interference is established between the capturing members 3, 4 and the flange, with the result that the beam anchor binds on the I-beam. The worker must then take the time and make the effort to apply a significant impact force to the anchor beam to dislodge or free the anchor beam from the I-beam, and thereby re-establish the mobility of the beam.